Process for manufacturing a cold rolled trip steel product

ABSTRACT

The present invention is related to a process comprising a cold rolling step, for the production of uncoated, electro-galvanised or hot dip galvanised TRIP steel products,
         hot rolling a slab of a specific composition, wherein the finishing rolling temperature is higher than the Ar3 temperature, to form a hot-rolled substrate,   cooling said substrate to a coiling temperature (CT) between 500° C. and 680° C.,   coiling said substrate at said coiling temperature,   pickling said substrate to remove the oxides,   cold rolling said substrate to obtain a reduction of thickness, with a minimum reduction of 40%.

FIELD OF THE INVENTION

The present invention is related to a steel composition comprising phosphor, to be used for the production of TRIP steel products. The invention is equally related to the process of production of said products, and to the end products themselves.

STATE OF THE ART

In the automotive industry there is a need for weight reduction, which implies the use of higher strength materials in order to be able to decrease the thickness of the parts without giving up safety and functional requirements. Ultra high strength steel (UHSS) sheet products and in particular TRIP steel products, showing a remarkable combination of high strength and good formability, can provide the solution for this problem. Additionally, an increased corrosion resistance of these steel sheet products by means of electro or hot dip galvanising, is frequently asked for.

Several documents are describing such UHSS products. EP-A-1096029 is related to the production of a tempered martensite TRIP steel, whose chemical composition is silicon-manganese based and contains (in wt %) 0.05-0.20% C, 0.3-1.8% Si and 1.0-3.0% Mn as well as one or more of the following additions (in wt %): 0.05-1% Cr+Mo, ≦0.003% B, 0.01-0.1% Ti+Nb+V and ≦0.01% Ca+REM. The cold rolling production process consists of three consecutive annealing steps. In the first step, the sheet is completely austenised during at least 5 seconds and subsequently rapidly cooled (>10° C./s) below the Ms (Martensite start) temperature in order to produce lath martensite. The second and third step are combined in a continuous annealing or galvanising line and consist of reheating the sheet in the intercritical region (Ac1<T<Ac3) during 5 to 120 seconds, cooling (>5° C./s) to 500° C. or lower and than subjecting the sheet to a galvanising or galvannealing treatment. There are two main drawbacks associated with this invention. The first one being the additional annealing step that is required to produce the lath martensite starting micro-structure. This extra process step will not only increase the total processing cost, but it will also complicate logistics as well as weldability at the entrance of the continuous annealing or hot dip galvanising line. Cracks that are initiated in the weld or heat affected zone, will easily propagate into the hard and brittle martensitic structure, leading to a high risk on complete rupture of the weld between two coils. The second major drawback is related to the rather high Si content in these steels. From about 0.5% Si on, these high Si contents are well known to provoke problems as to surface quality because of the presence of Si-oxides which after pickling create a surface with irregular and very high roughness. Moreover, in view of corrosion protection, hot dip galvanising of such a high Si-containing substrate in general leads to insufficient surface appearance for automotive applications, with moreover a very high risk on the presence of bare spots on the surface.

EP-A-0922782 also describes the production of a cold rolled Si—Mn based TRIP steel which contains (in wt %) 0.05-0.40% C, 1.0-3.0% Si, 0.6-3.0% Mn, 0.02-1.5% Cr, 0.01-0.20% P and 0.01-0.3% Al. As opposed to the previous invention, this product does not require the use of an additional annealing step. Cr is added to the analysis in order to retard the bainite formation and promote acicular ferrite and martensite formation as it is thought by the inventors that bainite is detrimental to the crushing behaviour in Si—Mn based TRIP steels. P is added to avoid pearlite formation and to increase the strength of the ferritic phase. The maximal P content is limited to 0.2% because of weldability. The high Si content in this invention will however again impair hot dip galvanisability resulting in an insufficient surface appearance and a very high risk on bare spots. The occurrence of red scale which is difficult to remove, on the hot strip, due to the higher Si content, is also expected to cause processing difficulties.

EP-A-0796928 describes the production of an Al-based Dual Phase steel which contains (in wt %) 0.05-0.3% C, 0.8-3.0% Mn, 0.4-2.5% Al and 0.01-0.2% Si. Additionally the steel can contain one of the following elements (in wt %) <0.05% Ti, <0.8% Cr, <0.5% Mo, <0.5% Ni, <0.05% Nb and <0.08% P. After cold rolling with a reduction rate higher than 40%, the material is intercritically annealed at temperatures between 740 and 850° C. and subsequently cooled at a cooling rate of 10 to 50 K/s to the Zn-bath temperature. As compared to both previous analyses, the latter, nearly Si-free analysis, readily allows the steel to be hot dip galvanised and avoids the formation of the detrimental red scale. However, Al, as opposed to Si, does not produce a strong solid solution strengthening effect. This implies the use of rather high Al-contents in order to reach medium high strength levels (e.g. Rm=700 MPa). These high Al levels are however known to cause stickers during continuous casting and impair weldability due to the presence of Al-oxides in the welded area. This is especially detrimental for the crash behaviour of welded structures. In order to avoid the casting problems an adapted very fine casting powder is required that could cause health problems. Steelmaking plants are therefore in general not willing to produce this kind of compositions because the workers have to carry masks and a lot of special precautions have to be taken.

EP-A-1170391 describes the production of a low carbon (<0.08 wt %), low silicon (<0.5 wt %) and low aluminium (<0.3 wt %) TRIP steel obtained by adding a nitriding step to the processing (0.03-2 wt % N). The Al and Si contents have to be kept low in order to avoid nitride precipitation and thus loss of free N. Furthermore the Si content is preferably lower than 0.2 wt % because of hot dip galvanisability. The carbon content is kept very low because of weldability and because of the fact that the presence of nitrogen in the steel also stabilises the retained austenite. This nitrogen is incorporated in the steel sheet either during or immediately after hot finish rolling, during recrystallisation annealing, during intercritical annealing or via a combination of one or more of these processes. All of them require the steel sheet to be held for 2 sec to 10 min. in an atmosphere containing not less than 2% ammonia in the temperature range 550-800° C. It is clear that this nitriding step makes processing a lot more difficult and requires complicated technical modifications to existing installations. At the moment this process is internationally not considered to be industrially feasible. Furthermore the very low alloying content of this steel grade, does not allow to reach tensile strength levels above 650 MPa.

U.S. Pat. No. 5,470,529 deals with the production of cold rolled TRIP steels based upon a wide variety of combined Al—Si analyses. The carbon content range is set as 0.05-0.3 wt %, but more preferably is 0.1-0.2 wt %. The Si-content is kept below 1.0 wt % in order to avoid red scale formation, but more preferably is in the range 0.2-0.9 wt %. Manganese is added in 0.005 to 4.0 wt %, but more preferably 0.5-2.0 wt %. As compared to traditional Si—Mn TRIP compositions, part of the Si is replaced by Al for various reasons. Like Si, Al also avoids cementite precipitation during bainitic holding. This enables to use lower Si-levels and thus avoid red scale formation. Furthermore the addition of Al increases the Ar3 temperature, leading to an increased carbon concentration in the austenite phase that is formed during intercritical annealing. This in turn stabilises the retained austenite and makes the steel less susceptible to stress-induced transformation in low stress regions, leading to an improved hole expansion ratio. Therefore the Al-range is set as 0.1-2.0 wt % and more preferably as 0.5-1.5 wt %. However, as Al and Si are both ferrite stabilizers, their sum is limited in order to avoid over-stabilizing the retained austenite. The Al+Si content should be in the range 0.5-3.0 wt % and more preferably in the range 1.5-2.5 wt %. In this invention P is considered as an incidental impurity that should be limited as much as possible. The P-limit is set at 0.1 wt % or less and preferably less than 0.02 wt %. Cu is added to the analysis to facilitate the removal of red scale, to improve the corrosion resistance of the as cold rolled product and to improve the wettability by molten Zn. Therefore the Cu-range is 0.1-2.0 wt % and more preferably 0.1-0.6 wt %. To avoid hot shortness problems when using Cu, Ni is added as well. For economics its content is limited to 1.0 wt % and preferably 0.5 wt %. The following constraints also apply: Ni(wt %)>Cu(wt %)/3 when Cu>0.5 wt % and Mn+Ni>0.5 wt %. Cr may be added as well to stabilise the retained austenite and to further improve corrosion resistance. It is added in the range 0.5-5.0 wt % and more preferably in the range 0.6-1.6 wt %. To increase tensile strength further Ti, Nb and V might be added as well. Their upper limit is preferably 0.05 wt % for Nb and Ti and 0.10 wt % for V. Although the maximum Si-content in this invention is limited to <1 wt % in order to avoid red scale formation, most of the cold rolled example steels have a Si-content in the range 0.5-1.1 wt %. The latter is considered to give rise to hot dip galvanising difficulties (bad wettability by molten Zn) and a deteriorated surface appearance (bare spots). None of these example steels contained micro-alloying additions as in high Si-TRIP steels, these are known to markedly increase the hot strip hardness, leading to strongly increased cold rolling forces. The low Si (0.2-0.4 wt %) example steels on the other hand showed a high yield stress (570-590 MPa) and only moderate ultimate tensile strength (700 MPa) and total elongation values (A50≦30%). In the latter steels no P was added additionally. A large disadvantage of these compositions is the necessity of adding Cu and Ni, elements which are considered as impurities in bulk flat carbon steel production. If a steelmaking plant has to cast this, extra logistic problems with scrap recycling occur. Moreover, the use of Ni, Cu and Cr makes the alloying cost much more expensive.

EP-A-1154028 describes the manufacturing of a P-alloyed low-Al, low-Si TRIP steel, which contains (in wt %): 0.06-0.17% C, 1.35-1.80% Mn, 0.35-0.50% Si, 0.02-0.12% P, 0.05-0.50% Al, max. 0.07% Nb, max. 0.2% V, max. 0.05% Ti, max. 30 ppm B and 100-350 ppm N. When the carbide forming elements Ti, Nb or V are added, the carbon content is preferably 0.16 wt %. The amount of residual austenite is limited to a maximum of 10%. The combination of low Si-content and rather low C-content, results in tensile strength values which are quite low (<600 MPa). When micro-alloying elements are added, the strength level is markedly improved (800 MPa), but elongations drop drastically (A80<17%). Elongation values are in all cases rather low, which can be explained by the limited addition of Al and C, rendering the retained austenite less stable.

L. Barbé et al have studied the influence of phosphorus additions to several TRIP compositions, ranging from pure Si-TRIP steels over combined Al—Si TRIP-steels to pure Al-TRIP steels (“Effect of phosphorus on the properties of a cold rolled and intercritically annealed TRIP-aided steel”, Int. Conf. on TRIP-Aided High Strength Ferrous Alloys, Ghent, Jun. 19-21, 2002). They found that the TRIP-aided steel composition which contained (in wt %) 0.24% C, 1.66% Mn, 0.6% Al, 0.4% Si and 0.073% P resulted in an excellent combination of mechanical properties (A80=28.4% and UTS=788 MPa). Lab-experiments performed by the inventors of the present invention have however shown that Al-additions as low as 0.6 wt %, render the obtained mechanical properties very sensitive to process parameter variations such as line speed and overageing temperature. This can lead to a non-compatibility between different galvanising lines (with e.g. different lengths of the levelling zone around 490-460° C.) or even to strongly thickness-dependent mechanical properties. This can be explained by the insufficient stabilisation of the retained austenite and the shift of the optimum overageing time to noticeably longer times due to the too low Al-additions.

O. Yakubovsky et al have studied the stress-strain behaviour and bake hardening behaviour of several TRIP compositions, ranging from pure Si-TRIP steels over combined Al—Si TRIP-steels to pure Al-TRIP steels (“Stress-strain behaviour and bake hardening of TRIP and TRIP-aided multiphase steels”, Int. Conf. on TRIP-Aided High Strength Ferrous Alloys, Ghent, Jun. 19-21, 2002). In all cases the carbon content was limited to (in wt %) 0.15% and the manganese content to 1.5%. Amongst the steels studied was also a (in wt %) 0.25-0.45% Si, 1.5-2.0% Al and 0.05-0.10% P TRIP steel. No mechanical properties were mentioned in the article for the latter composition. Based upon the tensile strength versus carbon content relationship that was established from industrial production and laboratory research by the inventors of the present invention, the proposed chemical composition is insufficiently alloyed in carbon to reach tensile strengths in the range 700-850 MPa. Furthermore the high Al-content requires the use of an adapted very fine casting powder that can give rise to health problems. Furthermore the weldability can be impaired due to the presence of Al-oxides in the welded area, a consequence of the high Al-contents.

S. Papaefthymiou et al. studied the microstructure development and mechanical behaviour of two Al—Si-TRIP steels that were uni-axially deformed to different strain levels directly after intercritical annealing (“Microstructure development and mechanical behaviour of Al-containing TRIP-steels”, Int. Conf. on TRIP-Aided High Strength Ferrous Alloys, Ghent, Jun. 19-21, 2002). As a consequence of this special treatment and because of the non-standard sample geometry, the mechanical properties mentioned in the article are not comparable. The two Al containing TRIP steels that were studied can be divided in a low-Al and a high-Al steel. These contain (in wt %): low-Al steel: 0.19% C, 1.5% Mn, 0.26% Si, 0.086% P and 0.52% Al; high-Al steel: 0.17% C, 1.46% Mn, 0.26% Si, 0.097% P and 1.81% Al. As already explained before, the low-Al steel will suffer from mechanical properties that are very sensitive to process parameter variations such as line speed and overageing temperature. This can lead to a non-compatibility between different galvanising lines or even to strongly thickness-dependent mechanical properties. The high-Al steel on the other hand again requires the use of an adapted casting powder that can give rise to health problems. Furthermore the weldability will be impaired due to the presence of Al-oxides in the welded area.

A. Pichler et al. (“Correlation between thermal treatment, retained austenite stability and mechanical properties of low-alloyed TRIP steels”, Int. Conf. on TRIP-Aided High Strength Ferrous Alloys, Ghent, Jun. 19-21, 2002) studied the influence of different annealing process parameters on the retained austenite stability and mechanical properties of a low-alloyed TRIP-steel that contained (in wt %): ˜0.2% C, ˜1.6% Si+Al, ˜1.5% Mn, <0.5% Cr+Mo, <0.04% P, <0.01% S and <0.05% Ti+Nb. As no specifications were given related to the Al/Si ratio in this analysis, it is not possible to conclude about the galvanisability. The mentioned P-content is insufficient for obtaining the desired mechanical properties (high tensile strength in combination with high elongation), whilst maintaining a good weldability (sufficient carbon content reduction).

AIMS OF THE INVENTION

It is the aim of the present invention to provide a high strength, low Si, high Al, P-alloyed TRIP steel composition for the production of steel products, produced by cold rolling that can readily be galvanised.

It is a further aim to propose a process for the manufacturing of such products, and to propose finally the products themselves, wherein said composition, said process and said products do not suffer from prior art problems.

SUMMARY OF THE INVENTION

The present invention is related to a cold rolled Al—Si P-alloyed TRIP steel composition intended to be used as uncoated, electro-galvanised or hot dip galvanised material. Said composition is characterised by the following contents:

-   -   C: between 1300 ppm and 2600 ppm     -   Mn: between 10000 ppm and 22000 ppm     -   Al: between 8000 ppm and 15000 ppm     -   Si: between 2000 ppm and 6000 ppm     -   P: between 400 and 1000 ppm     -   S: maximum 120 ppm     -   N: maximum 200 ppm     -   Ti: maximum 1000 ppm     -   Nb: maximum 1000 ppm     -   V: maximum 1000 ppm     -   B: maximum 10 ppm         the remainder being substantially iron and incidental         impurities.

The novelty and inventive step of this composition lies in the specific combination of elements P, Si, Al and C. In particular, adding P in excess of prior art levels, whilst limiting the maximum Si- and Al-content, allows to decrease the C-content for reaching a certain strength level, in combination with better weldability.

Three specific embodiments are related to the same chemical composition, but having three different subranges for carbon which are related to the strength level that is aimed at:

-   -   UTS (Ultimate Tensile strength) 590 MPa: carbon between 1300 ppm         and 1900 ppm. Two particular embodiments are characterized by a         carbon content of 1350 ppm to 1900 ppm and 1400 ppm to 1900 ppm         respectively.     -   UTS≧690 MPa: carbon between 1700 ppm and 2300 ppm     -   UTS≧780 MPa: carbon between 2000 ppm and 2600 ppm

Likewise three specific embodiments are related to the same subranges for carbon content which are related to the strength level that is aimed at, but having furthermore the following specific chemical composition:

-   -   Mn: between 13000 ppm and 22000 ppm     -   Al: between 8000 ppm and 14000 ppm     -   Si: between 2500 ppm and 4500 ppm     -   P: between 600 and 1000 ppm     -   S: maximum 120 ppm     -   N: maximum 150 ppm     -   Ti: maximum 200 ppm     -   Nb: maximum 100 ppm     -   V: maximum 100 ppm     -   B: maximum 5 ppm         the remainder being substantially iron and incidental         impurities.

Likewise three more specific embodiments are related to the same subranges for carbon content which are related to the strength level that is aimed at, but having furthermore the following specific sub-range for aluminium: 9000-1300 ppm. The subranges for the other alloying elements are kept the same as in paragraph [0017].

The present invention is equally related to a process for manufacturing a cold rolled TRIP steel product, comprising the steps of:

-   -   preparing a steel slab having a composition according to the         invention,     -   hot rolling said slab, wherein the finishing rolling temperature         is higher than the Ar3 temperature, to form a hot-rolled         substrate,     -   cooling said substrate to a coiling temperature (CT) between         500° C. and 680° C.,     -   coiling said substrate at said coiling temperature,     -   pickling said substrate to remove the oxides,     -   cold rolling said substrate to obtain a reduction of thickness,         with a minimum reduction of 40%.

According to a first embodiment, the process of the invention further comprises the steps of:

-   -   soaking said substrate at a temperature between 760° C. and 850°         C.,     -   cooling said substrate with a cooling rate higher than 2° C./s         to a temperature in the range 360° C. to 450° C.,     -   holding said substrate in said temperature range for a time less         than 700 s,     -   cooling said substrate to room temperature at a cooling rate         higher than 1° C./s.     -   subjecting said substrate to a skinpass reduction of maximum         1.5%.

According to a second embodiment, the process of the invention further comprises an electrolytic zinc coating step.

According to a third embodiment, the process of the invention further comprises the following processing steps after the cold rolling step:

-   -   soaking said substrate at a temperature between 760° C. and 850°         C.,     -   cooling said substrate with a cooling rate higher than 2° C./s         to the temperature of a Zn-bath,     -   holding said substrate in the temperature range between 490° C.         and 460° C. for less than 200 seconds.     -   hot dip galvanising said substrate in said Zn-bath,     -   cooling said substrate to room temperature at a cooling rate         higher than 2° C./s.

The process comprising a hot dip galvanising step may further comprise the step of subjecting said substrate to a skinpass reduction of maximum 1.5%.

The invention is equally related to a steel product produced according to the process of the invention and having a microstructure comprising 30-75% ferrite, 10-40% bainite, 0-20% retained austenite and possibly 0-10% martensite.

The invention is equally related to a steel product produced according to the process of the invention and having a carbon content between 1300 ppm and 1900 ppm. Said product has a yield strength between 320 MPa and 480 MPa, a tensile strength above 590 MPa, an elongation A80 higher than 26% and a n-value (this is the strain hardening coefficient, calculated between 10% and uniform elongation) higher than 0.2.

The invention is further related to a steel product produced according to the process of the invention and having a carbon content between 1700 and 2300 ppm. Said product has a yield strength between 350 MPa and 510 MPa, a tensile strength above 700 MPa, an elongation A80 higher than 24% and a n-value (calculated between 10% and uniform elongation) higher than 0.19.

The invention is further related to a steel product produced according to the process of the invention and having a carbon content between 2000 ppm and 2600 ppm. Said product has a yield strength between 400 MPa and 600 MPa, a tensile strength above 780 MPa, an elongation A80 higher than 22% and a n-value (calculated between 10% and uniform elongation) higher than 0.18.

The invention is also related to a steel product produced according to the process of the invention and having a carbon content between 2000 and 2600 ppm. Said product has a yield strength between 450 MPa and 700 MPa, a tensile strength above 980 MPa, an elongation A80 higher than 18% and a n-value (calculated between 10% and uniform elongation) higher than 0.14.

A steel product according to the invention may have a bake hardening BH2 higher than 40 MPa in both longitudinal and transversal directions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, a steel composition is proposed for the production of a P-alloyed Al—Si TRIP steel product. Application of the broadest chemical composition ranges which are indicated, will be able, in combination with the right process parameters, to result in products having a desired TRIP microstructure, good weldability as well as excellent mechanical properties, with very high values of the product of tensile strength and total elongation (this value is characteristic for a high energy absorption potential in case of a crash). The preferred ranges are related to more narrow ranges of mechanical properties, for example a guaranteed minimum tensile strength of 780 MPa, or to more stringent requirements on weldability (maximum of C-range, see next paragraph).

C: between 1300 ppm and 2600 ppm. A first preferred subrange is 1300-1900 ppm. A second preferred subrange is 1700-2300 ppm. A third preferred subrange is 2000-2600 ppm. The minimum carbon content per sub-range is needed in order to ensure the strength level as carbon is the most important element for the hardenability. The maximum of the claimed range per sub-range is related to weldability. The effect of carbon on mechanical properties is illustrated by exemplary composition A, E and F and reference compositions B, C and D (tables 1, 3-8). The effect of carbon content on spot weldability is illustrated by reference compositions B, C and D (table 2). Two specific subranges for carbon are characteristic for two specific embodiments: 1350-1900 ppm and 1400-1900 ppm. These subranges are aimed at ensuring an Ultimate Tensile Strength of at least 600 MPa.

Mn: between 10000 ppm and 22000 ppm, preferably between 13000-22000 ppm. Manganese acts as an austenite stabiliser and thus decreases the Ms temperature of the retained austenite. Furthermore Mn suppresses pearlite formation and also contributes to the overall strength level of the steel by solid solution hardening. Adding excess Mn results on the other hand in insufficient ferrite formation upon cooling from the soaking temperature and thus to insufficient carbon concentration in the retained austenite, rendering the latter less stable. Too much Mn will also increase the hardness of the weld and will enhance the formation of detrimental banded microstructures.

Al: between 8000 ppm and 15000 ppm, preferably between 8000-14000 ppm and most preferably between 9000-13000 ppm. Aluminium is added because, to an even stronger degree than Si, it is a ferrite stabiliser and thus enhances the ferrite formation during soaking and during cooling from the soaking temperature, thereby stabilising the retained austenite. The latter is stabilised even more by the fact that Al also suppresses the precipitation of carbon from the retained austenite during the overageing stage. Unlike Si, Al has no detrimental effect on galvanisability. Al-contents above 15000 ppm are however known to require the use of an adapted very fine casting powder that can cause health problems. Furthermore weldability can deteriorate due to the presence of Al-oxides in the welded area. A minimum Al content is however required to allow the material to be processed on different hot dip galvanising lines with different lengths of the levelling zones and to ensure a high process robustness.

Si: between 2000 ppm and 6000 ppm, preferably between 2500-4500 ppm. Silicon has essentially the same function as Al, albeit slightly less pronounced. That is: Si is a ferrite stabiliser and prevents carbide precipitation during the overageing stage, thereby stabilising the retained austenite at room temperature. Besides this, Si also contributes to the overall strength level of the steel by solid solution hardening. The maximum Si-content is however limited as Si is well known to provoke problems as to surface quality because of the presence of Si-oxides which after pickling create a surface with irregular and very high roughness. Moreover, in view of corrosion protection, hot dip galvanising of high Si-containing substrates in general leads to insufficient surface appearance for automotive applications, with moreover a high risk on the presence of bare spots on the surface.

P: between 400 ppm and 1000 ppm, preferably between 600-1000 ppm. Phosphorous is added primarily to allow the carbon content to be decreased to obtain improved weldability, while maintaining the same tensile strength level. Furthermore, P in combination with Si, is known to enhance the retained austenite stability by suppressing carbide precipitation during the overageing stage. In this respect P additions below 400 ppm do not allow a sufficiently large reduction of C-content. When adding more than 1000 ppm P, the risk on segregation defects increases and weldability is again deteriorated.

S: maximum 120 ppm. The S-content has to be limited because a too high inclusion level can deteriorate the formability.

N: maximum 200 ppm, preferably maximum 150 ppm otherwise too much AlN and/or TiN precipitates can form which are detrimental to formability.

Ti: maximum 1000 ppm, preferably below 200 ppm for products produced according to the present invention having a tensile strength below 980 MPa. Titanium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening. However for tensile strengths below 980 MPa, even without adding Ti, using the appropriate processing parameters, will result in the targeted mechanical properties per carbon sub-range and thus avoid an increase in analysis cost or extra processing difficulties (e.g. rolling forces).

Nb: maximum 1000 ppm, preferably below 100 ppm for products produced according to the present invention having a tensile strength below 980 MPa. Niobium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening. However for tensile strengths below 980 MPa, even without adding Nb, using the appropriate processing parameters, will result in the targeted mechanical properties per carbon sub-range and thus avoid an increase in analysis cost or extra processing difficulties (e.g. rolling forces).

V: maximum 1000 ppm, preferably below 100 ppm for products produced according to the present invention having a tensile strength below 980 MPa. Vanadium can be added in order to increase the tensile strength of the steel by grain refinement and precipitation strengthening. However for tensile strengths below 980 MPa, even without adding V, using the appropriate processing parameters, will result in the targeted mechanical properties per carbon sub-range and thus avoid an increase in analysis cost.

B: maximum 10 ppm, preferably maximum 5 ppm. Boron is avoided because of its detrimental influence on ferrite nucleation.

The present invention is equally related to the process for producing said steel product. This process comprises the steps of:

-   -   preparing a steel slab having a composition according to the         invention, such as defined above,     -   if necessary, reheating said slab to a temperature higher than         1000° C., preferably above 1200° C.,     -   hot rolling the slab, wherein the finishing rolling temperature         FT at the last stand of hot rolling is higher than the Ar3         temperature,     -   cooling to coiling temperature CT, preferably by continuous         cooling to the CT, typically at 40-50° C./s. Stepwise cooling         may be used as well,     -   hot rolling mill coiling of said substrate at a coiling         temperature CT comprised between 500° C. and 680° C., preferably         between 600° C. and 680° C. This temperature range is chosen so         as to create a hot band which is as soft as possible in order to         facilitate cold rolling,     -   pickling the substrate to remove the oxides,     -   cold rolling to obtain a reduction of thickness. The cold         rolling reduction is preferably higher than 40%.

According to a first embodiment of the invention, these steps are followed by an annealing treatment in a continuous annealing line, comprising the following steps:

-   -   soaking said pickled cold rolled substrate in a temperature         range between 760 and 850° C. in order to create a         microstructure that consists of ferrite and austenite. If the         soaking temperature is chosen above 850° C., the amount of         austenite formed will be too large, which leads to a less stable         retained austenite in the end product. Due to the reduced         austenite stability, a substantial part of it could also         transform to martensite during final cooling to room temperature         which deteriorates elongation properties. If, on the other hand,         the soaking temperature would be chosen too low, insufficient         austenite would be formed during soaking. This could lead to         over-stabilisation of the retained austenite, which again         deteriorates mechanical properties.     -   Cooling said substrate with a cooling rate higher than 2° C./s         to a holding temperature in the range 360° C. to 450° C. The         holding time in said temperature range is less than 700 seconds.         When the holding temperature would be chosen below 360° C., a         substantial part of the retained austenite will transform to         martensite, leading to a DP-like behaviour (high initial n-value         that decreases as a function of increasing strain) of the final         product. Holding temperatures above 450° C., will on the other         hand lead to a decomposition of the retained austenite by carbon         precipitation. This will again deteriorate elongation         properties.     -   Final cooling of said substrate to a temperature below 150° C.         with a cooling rate higher than 1° C./.     -   Finally said substrate can be subjected to a skinpass reduction         which is preferably in the range 0.3% to 1.5%.

A second preferred embodiment comprises the same processing steps mentioned above, but additionally also comprises an electrolytic zinc coating step.

According to a third embodiment of the invention, the cold rolling step is followed by an annealing treatment in a continuous hot dip galvanising line, comprising the following steps:

-   -   soaking said pickled cold rolled substrate in a temperature         range between 760 and 850° C. in order to create a         microstructure that consists of ferrite and austenite. If the         soaking temperature is chosen above 850° C., the amount of         austenite formed will be too large, which leads to a less stable         retained austenite in the end product. Due to the reduced         austenite stability, a substantial part of it could also         transform to martensite during final cooling to room temperature         which deteriorates elongation properties. If, on the other hand,         the soaking temperature would be chosen too low, insufficient         austenite would be formed during soaking. This could lead to         over-stabilisation of the retained austenite, which again         deteriorates mechanical properties.     -   cooling said substrate with a cooling rate higher than 2° C./s         to the temperature of the Zn-bath,     -   holding said substrate in the temperature range between 490° C.         and 460° C. for less than 200 seconds and preferably between 5         seconds and 80 seconds.     -   hot dip galvanising said substrate in said Zn-bath,     -   final cooling to room temperature at a cooling rate higher than         2° C./s.     -   Finally said substrate can be subjected to a skinpass reduction         which is preferably in the range 0.3% to 1.5%.

The thickness of the steel substrates of the invention after cold rolling can be lower than 1 mm according to the initial hot rolled sheet thickness and the capability of the cold rolling mill to perform the cold rolling at a sufficiently high level. Thus, thicknesses between 0.3 and 2.5 mm are feasible.

The resulting cold rolled product has a multiphase structure with 30-75% ferrite, 10-40% bainite, 0-20% retained austenite and possibly amounts of martensite (0-10%) present at room temperature. The amount of martensite at room temperature should however be limited in order to maintain an n-value behaviour (constant or increasing with strain) and mechanical properties that are characteristic for TRIP-steels. Specific mechanical properties as a function of processing parameter values are given in the examples.

The cold rolled non-temper rolled product showed in all cases a yield point elongation, which is typical for TRIP-steels and indicates that no or only very small amounts of martensite are present in the microstructure. This yield point elongation can be suppressed by temper rolling the final product. Small temper rolling reductions are sufficient to avoid the occurrence of a yield point elongation and temper rolling reductions above 1.5% should be avoided in order to prevent a too large yield strength increase.

The final cold rolled product furthermore preferably exhibits a constant or increasing n-value with increasing strain. This behaviour implies that the retained austenite is gradually transformed into martensite as the tensile test progresses thereby postponing the occurrence of necking, leading to an excellent combination of tensile strength and total elongation.

The robustness of TRIP steel products produced according to this invention is ensured by the minimum Al-content specified in the preferred Al-range: 8000-14000 ppm and most preferably in the range 9000-13000 ppm. Adding less Al will render the retained austenite less stable. This will increase the risk of loss of mechanical properties by austenite decomposition through carbon precipitation and on the other hand the less stable retained austenite will more easily transform into martensite during straining, limiting the formability of the material. Adding less Al will also retard the bainite transformation kinetics. As a consequence the mechanical properties will become more dependent on processing conditions such as line speed and overageing temperature as well on the actual line lay-out (short or long overageing section). Using an Al-content within the preferred range, avoids such line dependency and loss of robustness.

Concerning weldability of the obtained cold rolled product, the addition of phosphorus allows to decrease the carbon content as compared to P-free Al—Si TRIP steels of the same tensile strength level. As in the investigated carbon range, weldability is improved by lowering the carbon content, such a carbon content reduction by P-addition can be considered as the main advantage of the current invention.

The different drawbacks described above as to the compositions described in state of the art publications are not encountered when the composition of the present invention is applied:

-   -   Si is limited in order to ensure the hot dip galvanisability.         The surface appearance of the hot dip galvanised cold rolled         steel of the present invention is sufficient for automotive         unexposed applications whereas substrates with higher         Si-contents in general lead to insufficient surface appearance         for automotive applications, with moreover a much higher risk on         the presence of bare spots on the surface.     -   Si is furthermore limited in order to avoid the need of hot         charging the slabs to prevent crack formation.     -   The presence of red scale on the hot rolled substrate's surface         is also avoided by limiting the Si-content.     -   A small amount of Si, which is compatible with hot dip         galvanising, is added to more easily reach the targeted tensile         strength levels. In comparison with Si-free Al-TRIP steels, this         allows to use a lower C-content or lower Al-content.     -   The maximum Al-content is limited to avoid the use of an adapted         very fine casting powder that could cause health problems.         Furthermore in the case of high Al-contents (>1.5%) weldability         could deteriorate due to the presence of Al-oxides in the welded         area. A minimum Al-content however ensures a high process         robustness and less sensitivity to changes in line speed,         overageing temperature and lay-out of the continuous annealing         or hot dip galvanising line.     -   For a given tensile strength level, the C content is limited as         compared to other Al—Si TRIP steels in order to improve         weldability, by the addition of P and without the need of         micro-alloying.     -   No Ni, Cu or Cr are added to the steel products of the         invention. This avoids logistic problems with scrap recycling         and reduces the cost of the analysis.

EXAMPLES 1. Example Compositions

Table 1 shows examples of compositions of laboratory castings of the P-alloyed Al—Si TRIP steel product according to the present invention (codes A, E and F), and of reference compositions (B, C and D) having either a C-content which is higher than the claimed range and/or no intentionally added phosphor. Laboratory thermal cycle simulations and tensile tests were performed to obtain the mechanical properties of the test specimens of these example compositions. It is to be noted that in what follows, all mentioned tensile test mechanical properties are measured according to the standard EN10002-1.

1.1 Cold Rolled and Continuously Annealed Product

The processing steps were:

-   -   Casting,     -   Reheating at 1250° C. for 1 hour,     -   Hot rolling in the austenite region to a final thickness of 3.5         mm,     -   Water cooling to the coiling temperature of 600° C.,     -   Cold rolling to a final thickness of 1 mm,

After the previous processing steps, tensile specimens with 80 mm gauge length parallel to the rolling direction were machined out of the sheets. These specimens were heat treated in two salt baths following a two-step thermal cycle. After annealing in the intercritical region (IAT) for a given time (IAt), the samples were quenched in a salt bath at a lower temperature and isothermally held (BHT) for a given time (BHt). The mechanical properties are listed in table 3 and are conform the specifications for a TRIP700 grade. No temper rolling was applied. When comparing the mechanical properties mentioned in table 3 (example composition A), with those mentioned in table 5 (in particular reference composition C), it becomes clear that the addition of P in the steel of the present invention clearly allows to reduce the carbon content with 500 ppm while maintaining the tensile strength level. This C-reduction will seriously improve weldability (cfr. Table 2: difference between 0.25 C and 0.20 C). Table 7 contains the mechanical properties obtained after applying several continuous annealing simulations on steel samples of compositions E and F. Looking at the data in table 5 and 7 (in particular E compared to B), it is clear that the tensile strength is even higher for the composition of the invention, as compared to the reference composition which has 600 ppm more carbon and no intentionally added phosphor.

1.2 Cold Rolled and Hot Dip Galvanised Product

The processing steps were:

-   -   Casting,     -   Reheating at 1250° C. for 1 hour,     -   Hot rolling in the austenite region to a final thickness of 3.5         mm,     -   Water cooling to the coiling temperature of 600° C.,     -   Cold rolling to a final thickness of 1 mm.

After the previous processing steps, tensile specimens with 80 mm gauge length parallel to the rolling direction were machined out of the sheets. These specimens were heat treated in two salt baths following a two-step thermal cycle. After annealing in the intercritical region, the samples were quenched in a salt bath at a lower temperature and isothermally held. The temperatures and holding times were adapted in order to simulate closely the process steps, including a hot dip galvanizing step. The mechanical properties are listed in table 4 and clearly illustrate that the current invention leads to a very robust hot dip galvanized TRIP700 product: a broad variation of process parameters (IAT, Iat, BHT, BHt) all result in products with comparable mechanical properties. No temper rolling was applied. When comparing the mechanical properties mentioned in table 4 (example composition A), with those mentioned in table 6 (in particular reference composition C), it becomes clear that the addition of P in the steel of the present invention clearly allows to reduce the carbon content with 500 ppm while maintaining the tensile strength level. This C-reduction will seriously improve weldability (cfr. Table 2).

Table 8 contains the mechanical properties obtained after applying several hot dip galvanising simulations on steel samples of compositions E and F. Looking at the data in table 6 and 8 (in particular E compared to B), it is clear that the tensile strength is even higher for the composition of the invention, as compared to the reference composition which has 600 ppm more carbon and no intentionally added phosphor.

TABLE 1 Compositions (ppm) of Al—Si TRIP steels. Compositions A, E, F according to the invention, B, C and D are reference compositions. Code C Mn Al Si P S N Ti Nb V A 2000 16200 9000 3000 850 20 69 20 20 30 B 3000 15000 12000 3000 173 60 43 47 30 43 C 2500 15000 12000 3000 167 55 32 37 26 45 D 2000 15000 10000 3000 223 32 48 61 38 55 E 2400 15100 11200 2700 820 10 100 50 F 2200 15000 13400 4400 730 10 50 50

TABLE 2 Average of max. force measured at Imax in cross-tensile test on spot welds according to AFNOR-A87001. Code Fmax at Imax (N) B 6906 C 7489 D 9608

TABLE 3 Mechanical properties of the cold rolled and continuously annealed P-alloyed Al—Si TRIP steel, composition A, according to the present invention. No temper rolling applied. Thickness 1 mm. IAT BHT Rp0.2 Rm Au A80 Code (° C.) IAt (s) (° C.) BHt (s) (MPa) (MPa) (%) (%) A 770 60 400 120 486 708 24.7 30.9

TABLE 4 Mechanical properties of the cold rolled and hot dip galvanised P-alloyed Al—Si TRIP steel, composition A, according to the present invention. No temper rolling applied. Thickness 1 mm. IAT IAt BHT BHt Rp0.2 Rm Au A80 (° C.) (s) (° C.) (s) (MPa) (MPa) (%) (%) Re/Rm RmxA80 770 73 460 20 411 738 23.6 28.6 0.557 21107 490 408 756 22.6 28.0 0.540 21168 800 460 436 756 24.2 29.6 0.577 22378 490 428 775 22.5 28.1 0.552 21778 830 460 427 746 24.6 30.2 0.572 22529 490 434 761 22.4 28.1 0.570 21384 770 52 460 14 419 734 24.3 29.4 0.571 21580 490 419 754 22.5 27.2 0.556 20509 800 460 432 755 23.7 28.8 0.572 21744 490 427 760 23.1 28.3 0.562 21508 830 460 427 748 22.7 27.8 0.571 20794 490 428 771 21.7 27.0 0.555 20817 770 36 460 10 423 701 24.7 30.3 0.603 21240 490 419 726 23.8 28.7 0.577 20836 800 460 433 755 23.9 28.9 0.574 21820 490 426 743 22.0 26.8 0.573 19912 830 460 441 768 23.6 28.9 0.574 22195 490 449 765 23.0 27.9 0.587 21344 (BH = bainitic holding = holding before passing through the zinc bath)

TABLE 5 Mechanical properties of the cold rolled and continuously annealed reference Al—Si TRIP steels. Thickness SKP Re (MPa) Rm (MPa) A80 (%) Code (mm) (%) RD TD RD TD RD TD B 1 0.50 508 499 737 726 34.0 32.0 1.5 0.75 508 500 732 746 31.0 27.0 1.6 1.00 524 515 737 745 31.0 29.0 C 0.7 0.50 502 499 713 717 34.3 32.1 1.45 0.80 507 496 692 700 34.2 31.5 D 0.85 0.70 483 463 664 668 27.8 29.2 1.22 0.70 451 451 647 657 30.1 29.9 1.47 0.75 466 457 650 658 28.5 27.7 (RD: rolling direction; TD: transverse direction)

TABLE 6 Mechanical properties of the hot dip galvanised reference Al—Si TRIP steels. Thickness SKP Re (MPa) Rm (MPa) A80 (%) Code (mm) (%) RD TD RD TD RD TD B 1.00 0.5 490 482 800 803 32.1 30.1 1.2 0.7 483 485 802 810 32.7 30.7 C 1.00 0.6 458 449 727 732 34.6 35.4 1.45 0.9 472 462 732 727 33.2 31.4 D 1.49 1.0 446 438 652 647 33.8 30.9 1.23 1.1 451 456 673 675 32.8 33.0 (RD: rolling direction; TD: transverse direction)

TABLE 7 Mechanical properties of the cold rolled and continuously annealed P-alloyed Al—Si TRIP steel, compositions E and F, according to the present invention. No temper rolling applied. Thickness 1 mm. IAT IAt BHT Rp0.2 Rm A80 Code (° C.) (s) (° C.) BHt (MPa) (MPa) (%) Re/Rm RmxA80 E 800 209 375 448 595 820 26.1 0.726 21402 425 593 795 33.2 0.746 26394 125 375 280 562 834 26.9 0.674 22434 425 589 798 27.3 0.738 21785 F 800 209 375 448 597 837 27.5 0.713 23017 425 582 819 32 0.710 26208 125 375 280 569 855 27.3 0.665 23341 425 589 820 32.4 0.718 26568

TABLE 8 Mechanical properties of the cold rolled and hot dip galvanised P-alloyed Al—Si TRIP steel, compositions E and F, according to the present invention. No temper rolling applied. Thickness 1 mm. IAT IAt BHT BHt Rp0.2 Rm A80 Code (° C.) (s) (° C.) (s) (MPa) (MPa) (%) Re/Rm RmxA80 E 800 73 460 20 496 862 26.9 0.575 23188 490 481 901 24.6 0.534 22165 45 460 65 544 858 28.3 0.634 24281 490 528 872 24.3 0.606 21190 36 460 10 555 799 24.4 0.695 19496 490 526 881 27.9 0.597 24580 22 460 32 490 872 23.6 0.562 20579 490 549 850 27.8 0.646 23630 F 800 73 460 20 487 924 25.1 0.527 23192 490 437 971 22.1 0.450 21459 45 460 65 551 830 29.9 0.664 24817 490 509 938 25.3 0.543 23731 36 460 10 552 855 27.7 0.646 23684 490 480 922 24.7 0.521 22773 22 460 32 480 920 24.9 0.522 22908 490 532 881 25.8 0.604 22730 (BH = bainitic holding = holding before passing through the zinc 

1. A process for manufacturing a cold rolled TRIP steel product, comprising the steps of: preparing a steel slab having the following composition: C: between 1300 ppm and 2600 ppm, Mn: between 10000 ppm and 22000 ppm, Al: between 8000 ppm and 15000 ppm, Si: between 2000 ppm and 6000 ppm, P: between 400 and 1000 ppm, S: maximum 120 ppm, N: maximum 200 ppm, Ti: maximum 1000 ppm, Nb: maximum 1000 ppm, V: maximum 1000 ppm, B: maximum 10 ppm. the remainder being substantially iron and incidental impurities, hot rolling said slab, wherein the finishing rolling temperature is higher than the Ar3 temperature, to form a hot-rolled substrate, cooling said substrate to a coiling temperature (CT) between 500° C. and 680° C., coiling said substrate at said coiling temperature, pickling said substrate to remove the oxides, cold rolling said substrate to obtain a reduction of thickness, with a minimum reduction of 40%.
 2. The process according to claim 1, further comprising the steps of: soaking said substrate at a temperature between 760° C. and 850° C., cooling said substrate with a cooling rate higher than 2° C./s to a temperature in the range 360° C. to 450° C., holding said substrate in said temperature range for a time less than 700 s, cooling said substrate to room temperature at a cooling rate higher than 1° C./s. subjecting said substrate to a skinpass reduction of maximum 1.5%.
 3. The process according to claim 2, further comprising an electrolytic zinc coating step.
 4. The process according to claim 1, further comprising the following processing steps: soaking said substrate at a temperature between 760° C. and 850° C., cooling said substrate with a cooling rate higher than 2° C./s to the temperature of a Zn-bath, holding said substrate in the temperature range between 490° C. and 460° C. for less than 200 seconds, hot dip galvanising said substrate in said Zn-bath, cooling said substrate to room temperature at a cooling rate higher than 2° C./s.
 5. The process according to claim 4, further comprising the step of subjecting said substrate to a skinpass reduction of maximum 1.5%.
 6. The process according to claim 1, wherein the carbon content of said composition is between 1300 ppm and 1900 ppm.
 7. The process according to claim 1, wherein the carbon content of said composition is between 1350 ppm and 1900 ppm.
 8. The process according to claim 1, wherein the carbon content of said composition is between 1400 ppm and 1900 ppm.
 9. The process according to claim 1, wherein the carbon content of said composition is between 1700 ppm and 2300 ppm.
 10. The process according to claim 1, wherein the carbon content of said composition is between 2000 ppm and 2600 ppm.
 11. The process according to claim 6, wherein said composition comprises: Mn: between 13000 ppm and 22000 ppm, Al: between 8000 ppm and 14000 ppm, Si: between 2500 ppm and 4500 ppm, P: between 600 and 1000 ppm, S: maximum 120 ppm, N: maximum 150 ppm, Ti: maximum 200 ppm, Nb: maximum 100 ppm, V: maximum 100 ppm, B: maximum 5 ppm.
 12. The process according to claim 11, wherein the aluminium content of said composition is between 9000 ppm and 13000 ppm. 